Why are propellers de-iced before the engines are started, and does ice on the fuselage affect the flight characteristics of the plane?

Question

I recently flew on a flight out of Montreal on a DHC8 turboprop. The plane had been parked at the airport overnight during a winter storm, and as I walked out to the plane, there were visible icicles accumulated on the wings. There was also a layer of ice completely covering the windows on one side of the plane, presumably because of the direction of the prevailing wind during the storm.

The plane was successfully de-iced and took off without a problem. However, I had some questions about the de-icing process as I observed it from inside the plane.

1. The ground crew had to de-ice the propellers manually before the engines could be started. Is this mainly because of the risk of ice ingestion in the engines, or is there a problem with the propeller being imbalanced because of uneven weight? Or both? Or neither?

2. The de-icing by the trucks seemed to focus on the wings, and it looked like there was still some ice left on the exterior windows when we left the de-icing pad. My understanding of icing is that ice on the wings is extra-bad (I remember a fatal crash happened near me due to wing ice when I was growing up), but does ice on the fuselage affect the flight characteristics of the plane? Or is it not really a concern?

Answer 1

The props are done before starting because you need to make sure the blades and spinners are fully cleaned off while they are stationary. Otherwise, they'd vibrate like hell when starting and shed bits of ice all over. Also, if you just sprayed the props while running the engines would ingest a lot of glycol, which, if it doesn't make the engine flame out, can gum things up and cause cabin smells as cooked fluid makes its way into the air conditioning system through the compressor bleed system (a lot of turn-backs from "cabin smoke" reports on departure are from the stink from cooked glycol getting into the cabin via the bleeds).

Even done stationary, some fluid will make its way into the engine and it's typical to run the engines at a moderate power setting after the treatment for a couple minutes to try to purge whatever glycol residue is in the compressor before opening the bleeds.

Fuselages are deiced depending on the airplane and the conditions. On some jets, especially those with tail mounted engines, the upper fuselage is treated as if it was a flying surface and must be clear of ice and frost, even small amounts (it varies with the regulatory jurisdiction). Otherwise it will depend on whether there is any significant accumulation, mostly for weight considerations.

If there is an obvious layer of snow/ice on the fuselage, it will be removed... mostly; don't expect a perfect job and they won't normally shoot down as far as the windows. If it's just morning frost, the fuselage may be left alone although the wings and tail must still be done (ANY frost on leading edges can be fatal). The Capt is in communication with the deicing personnel working the boom and will give instructions on what is to be covered, and what to avoid (like APU air inlet in the tail, or heat exchanger inlets, etc.), whether to use Type I only or Type I and II/III/IV (Type one is for melting/removal and II/III/IV is for follow-on residual protection for a limited time).

A deice with follow on residual anti-ice protection costs the airline a couple thousand bucks per flight so avoiding unnecessary fluid application is important in the long run.

Answer 2

1. Ice flying off the propellers can damage something, or someone. Also unbalances the propeller assembly overall, leading to vibrations.

2. Little bit of ice left on the fuselage is not bad, only causes some extra drag until it sublimates off, and of course a little extra weight. Ice Will impact the lift created by the wings, that is very bad, unless the ice is along the wing/fuselage joint, where it disrupt smooth airflow, altho probably just creating some more drag until it sublimates off.

Answer 3

Ice on airplanes causes two main problems.

1. It adds weight and drag.
2. It changes the shape of the aerodynamic surfaces.

Problem 1 is fairly obvious. How serious a problem it is depends on the airplane and how fully loaded it is. If it is a problem, then ice needs to be removed from any or all surfaces to reduce the problem to an acceptable level. If the plane is sufficiently over-powered or under its maximum gross weight, then this may be ignored.

Problem 2 is the main reason for deicing. The shape of the wings, propellers, and tail are carefully designed to achieve the desired airplane performance. (It may not be obvious, but propellers have an airfoil shape just as wings do.) Any change in shape is inevitably detrimental. If the shape of the propellers is affected by ice, then engine performance will suffer. If the shape of the wings is affected by ice, then lift will suffer. If the shape of the tail is affected by ice, stability and maneuverability will suffer.